Remove non-functional code for unloading loadable modules.

The code for unloading a library has been commented-out for over 12
years, ever since commit 602a9ef, and we're
no closer to supporting it now than we were back then.

Nathan Bossart, reviewed by Michael Paquier and by me.

Discussion: http://postgr.es/m/Ynsc9bRL1caUSBSE@paquier.xyz

Author: robertmhaas

contrib/auto_explain/auto_explain.c

@@ -77,7 +77,6 @@ static ExecutorFinish_hook_type prev_ExecutorFinish = NULL;
 static ExecutorEnd_hook_type prev_ExecutorEnd = NULL;
 
 void		_PG_init(void);
-void		_PG_fini(void);
 
 static void explain_ExecutorStart(QueryDesc *queryDesc, int eflags);
 static void explain_ExecutorRun(QueryDesc *queryDesc,
@@ -244,19 +243,6 @@ _PG_init(void)
 	ExecutorEnd_hook = explain_ExecutorEnd;
 }
 
-/*
- * Module unload callback
- */
-void
-_PG_fini(void)
-{
-	/* Uninstall hooks. */
-	ExecutorStart_hook = prev_ExecutorStart;
-	ExecutorRun_hook = prev_ExecutorRun;
-	ExecutorFinish_hook = prev_ExecutorFinish;
-	ExecutorEnd_hook = prev_ExecutorEnd;
-}
-
 /*
  * ExecutorStart hook: start up logging if needed
  */

contrib/passwordcheck/passwordcheck.c

@@ -33,7 +33,6 @@ static check_password_hook_type prev_check_password_hook = NULL;
 #define MIN_PWD_LENGTH 8
 
 extern void _PG_init(void);
-extern void _PG_fini(void);
 
 /*
  * check_password
@@ -149,13 +148,3 @@ _PG_init(void)
 	prev_check_password_hook = check_password_hook;
 	check_password_hook = check_password;
 }
-
-/*
- * Module unload function
- */
-void
-_PG_fini(void)
-{
-	/* uninstall hook */
-	check_password_hook = prev_check_password_hook;
-}

contrib/pg_stat_statements/pg_stat_statements.c

@@ -305,7 +305,6 @@ static bool pgss_save;			/* whether to save stats across shutdown */
 /*---- Function declarations ----*/
 
 void		_PG_init(void);
-void		_PG_fini(void);
 
 PG_FUNCTION_INFO_V1(pg_stat_statements_reset);
 PG_FUNCTION_INFO_V1(pg_stat_statements_reset_1_7);
@@ -481,23 +480,6 @@ _PG_init(void)
 	ProcessUtility_hook = pgss_ProcessUtility;
 }
 
-/*
- * Module unload callback
- */
-void
-_PG_fini(void)
-{
-	/* Uninstall hooks. */
-	shmem_startup_hook = prev_shmem_startup_hook;
-	post_parse_analyze_hook = prev_post_parse_analyze_hook;
-	planner_hook = prev_planner_hook;
-	ExecutorStart_hook = prev_ExecutorStart;
-	ExecutorRun_hook = prev_ExecutorRun;
-	ExecutorFinish_hook = prev_ExecutorFinish;
-	ExecutorEnd_hook = prev_ExecutorEnd;
-	ProcessUtility_hook = prev_ProcessUtility;
-}
-
 /*
  * shmem_startup hook: allocate or attach to shared memory,
  * then load any pre-existing statistics from file.

doc/src/sgml/xfunc.sgml

@@ -1978,28 +1978,16 @@ PG_MODULE_MAGIC;
    <indexterm zone="xfunc-c-dynload">
     <primary>_PG_init</primary>
    </indexterm>
-   <indexterm zone="xfunc-c-dynload">
-    <primary>_PG_fini</primary>
-   </indexterm>
    <indexterm zone="xfunc-c-dynload">
     <primary>library initialization function</primary>
    </indexterm>
-   <indexterm zone="xfunc-c-dynload">
-    <primary>library finalization function</primary>
-   </indexterm>
 
    <para>
-    Optionally, a dynamically loaded file can contain initialization and
-    finalization functions.  If the file includes a function named
+    Optionally, a dynamically loaded file can contain an initialization
+    function.  If the file includes a function named
     <function>_PG_init</function>, that function will be called immediately after
     loading the file.  The function receives no parameters and should
-    return void.  If the file includes a function named
-    <function>_PG_fini</function>, that function will be called immediately before
-    unloading the file.  Likewise, the function receives no parameters and
-    should return void.  Note that <function>_PG_fini</function> will only be called
-    during an unload of the file, not during process termination.
-    (Presently, unloads are disabled and will never occur, but this may
-    change in the future.)
+    return void.  There is presently no way to unload a dynamically loaded file.
    </para>
 
   </sect2>

src/backend/postmaster/pgarch.c

@@ -802,7 +802,7 @@ HandlePgArchInterrupts(void)
 			 * Ideally, we would simply unload the previous archive module and
 			 * load the new one, but there is presently no mechanism for
 			 * unloading a library (see the comment above
-			 * internal_unload_library()).  To deal with this, we simply restart
+			 * internal_load_library()).  To deal with this, we simply restart
 			 * the archiver.  The new archive module will be loaded when the new
 			 * archiver process starts up.
 			 */

src/backend/utils/fmgr/dfmgr.c

@@ -37,9 +37,8 @@
 #include "utils/hsearch.h"
 
 
-/* signatures for PostgreSQL-specific library init/fini functions */
+/* signature for PostgreSQL-specific library init function */
 typedef void (*PG_init_t) (void);
-typedef void (*PG_fini_t) (void);
 
 /* hashtable entry for rendezvous variables */
 typedef struct
@@ -79,7 +78,6 @@ char	   *Dynamic_library_path;
 static void *internal_load_library(const char *libname);
 static void incompatible_module_error(const char *libname,
 									  const Pg_magic_struct *module_magic_data) pg_attribute_noreturn();
-static void internal_unload_library(const char *libname);
 static bool file_exists(const char *name);
 static char *expand_dynamic_library_name(const char *name);
 static void check_restricted_library_name(const char *name);
@@ -154,9 +152,6 @@ load_file(const char *filename, bool restricted)
 	/* Expand the possibly-abbreviated filename to an exact path name */
 	fullname = expand_dynamic_library_name(filename);
 
-	/* Unload the library if currently loaded */
-	internal_unload_library(fullname);
-
 	/* Load the shared library */
 	(void) internal_load_library(fullname);
 
@@ -179,6 +174,11 @@ lookup_external_function(void *filehandle, const char *funcname)
  * loaded.  Return the pg_dl* handle for the file.
  *
  * Note: libname is expected to be an exact name for the library file.
+ *
+ * NB: There is presently no way to unload a dynamically loaded file.  We might
+ * add one someday if we can convince ourselves we have safe protocols for un-
+ * hooking from hook function pointers, releasing custom GUC variables, and
+ * perhaps other things that are definitely unsafe currently.
  */
 static void *
 internal_load_library(const char *libname)
@@ -400,71 +400,6 @@ incompatible_module_error(const char *libname,
 			 errdetail_internal("%s", details.data)));
 }
 
-/*
- * Unload the specified dynamic-link library file, if it is loaded.
- *
- * Note: libname is expected to be an exact name for the library file.
- *
- * XXX for the moment, this is disabled, resulting in LOAD of an already-loaded
- * library always being a no-op.  We might re-enable it someday if we can
- * convince ourselves we have safe protocols for un-hooking from hook function
- * pointers, releasing custom GUC variables, and perhaps other things that
- * are definitely unsafe currently.
- */
-static void
-internal_unload_library(const char *libname)
-{
-#ifdef NOT_USED
-	DynamicFileList *file_scanner,
-			   *prv,
-			   *nxt;
-	struct stat stat_buf;
-	PG_fini_t	PG_fini;
-
-	/*
-	 * We need to do stat() in order to determine whether this is the same
-	 * file as a previously loaded file; it's also handy so as to give a good
-	 * error message if bogus file name given.
-	 */
-	if (stat(libname, &stat_buf) == -1)
-		ereport(ERROR,
-				(errcode_for_file_access(),
-				 errmsg("could not access file \"%s\": %m", libname)));
-
-	/*
-	 * We have to zap all entries in the list that match on either filename or
-	 * inode, else internal_load_library() will still think it's present.
-	 */
-	prv = NULL;
-	for (file_scanner = file_list; file_scanner != NULL; file_scanner = nxt)
-	{
-		nxt = file_scanner->next;
-		if (strcmp(libname, file_scanner->filename) == 0 ||
-			SAME_INODE(stat_buf, *file_scanner))
-		{
-			if (prv)
-				prv->next = nxt;
-			else
-				file_list = nxt;
-
-			/*
-			 * If the library has a _PG_fini() function, call it.
-			 */
-			PG_fini = (PG_fini_t) dlsym(file_scanner->handle, "_PG_fini");
-			if (PG_fini)
-				(*PG_fini) ();
-
-			clear_external_function_hash(file_scanner->handle);
-			dlclose(file_scanner->handle);
-			free((char *) file_scanner);
-			/* prv does not change */
-		}
-		else
-			prv = file_scanner;
-	}
-#endif							/* NOT_USED */
-}
-
 static bool
 file_exists(const char *name)
 {

src/backend/utils/fmgr/fmgr.c

@@ -582,20 +582,6 @@ record_C_func(HeapTuple procedureTuple,
 	entry->inforec = inforec;
 }
 
-/*
- * clear_external_function_hash: remove entries for a library being closed
- *
- * Presently we just zap the entire hash table, but later it might be worth
- * the effort to remove only the entries associated with the given handle.
- */
-void
-clear_external_function_hash(void *filehandle)
-{
-	if (CFuncHash)
-		hash_destroy(CFuncHash);
-	CFuncHash = NULL;
-}
-
 
 /*
  * Copy an FmgrInfo struct

src/include/fmgr.h

@@ -705,7 +705,6 @@ extern bytea *OidSendFunctionCall(Oid functionId, Datum val);
  * Routines in fmgr.c
  */
 extern const Pg_finfo_record *fetch_finfo_record(void *filehandle, const char *funcname);
-extern void clear_external_function_hash(void *filehandle);
 extern Oid	fmgr_internal_function(const char *proname);
 extern Oid	get_fn_expr_rettype(FmgrInfo *flinfo);
 extern Oid	get_fn_expr_argtype(FmgrInfo *flinfo, int argnum);

src/pl/plpgsql/src/plpgsql.h

@@ -1100,8 +1100,6 @@ typedef struct PLpgSQL_execstate
  * variable "PLpgSQL_plugin" and set it to point to a PLpgSQL_plugin struct.
  * Typically the struct could just be static data in the plugin library.
  * We expect that a plugin would do this at library load time (_PG_init()).
- * It must also be careful to set the rendezvous variable back to NULL
- * if it is unloaded (_PG_fini()).
  *
  * This structure is basically a collection of function pointers --- at
  * various interesting points in pl_exec.c, we call these functions

src/test/modules/delay_execution/delay_execution.c

@@ -36,9 +36,8 @@ static int	post_planning_lock_id = 0;
 /* Save previous planner hook user to be a good citizen */
 static planner_hook_type prev_planner_hook = NULL;
 
-/* Module load/unload functions */
+/* Module load function */
 void		_PG_init(void);
-void		_PG_fini(void);
 
 
 /* planner_hook function to provide the desired delay */
@@ -97,10 +96,3 @@ _PG_init(void)
 	prev_planner_hook = planner_hook;
 	planner_hook = delay_execution_planner;
 }
-
-/* Module unload function (pro forma, not used currently) */
-void
-_PG_fini(void)
-{
-	planner_hook = prev_planner_hook;
-}